Subject: bug#59887: pcase vs. pcase-let: Underscore in backquote-style patterns

From: Michael Heerdegen <[email-masked]>
Date: Tue, 13 Dec 2022 02:17:56 +0100

hokomo <[email-masked]> writes:

> Your quote above made everything clear, but I completely missed it
> since I was reading the Emacs Lisp manual's explanation [1] rather
> than pcase-let's docstring. Maybe it would be beneficial to include
> the above quote in the manual as well.

> <https://www.gnu.org/software/emacs/manual/html_node/elisp/Destructuring-with-pcase-Patterns.html>

That says:

|    The macros described in this section use ‘pcase’ patterns to perform
| destructuring binding.  The condition of the object to be of compatible
| structure means that the object must match the pattern, because only
| then the object’s subfields can be extracted.  For example:
| 
|        (pcase-let ((`(add ,x ,y) my-list))
|          (message "Contains %S and %S" x y))
| 
| does the same as the previous example, except that it directly tries to
| extract ‘x’ and ‘y’ from ‘my-list’ without first verifying if ‘my-list’
| is a list which has the right number of elements and has ‘add’ as its
| first element.  The precise behavior when the object does not actually
| match the pattern is undefined, although the body will not be silently
| skipped: either an error is signaled or the body is run with some of the
| variables potentially bound to arbitrary values like ‘nil’.

That explains the same thing quite broadly.  Maybe you did not notice
the implications when you first read it?  I dunno, I'm not that good in
writing documentation, but I can't find something to add from what we
had discussed that would not be redundant.

Or should we maybe just warn about the possible pitfall a bit more
offensively?

Michael.